Value documents are understood here to be card- or in particular sheet-shaped objects that represent for example a monetary value or an authorization and/or are not to be producible at will by unauthorized persons. They therefore have features that are not easy to produce, in particular to copy, whose presence is an indication of authenticity, i.e. production by an authorized body. Important examples of such value documents are chip cards, coupons, vouchers, checks and in particular bank notes.
Value documents are often analyzed optically for recognition of their type and/or their state and/or for a check of authenticity. It is fundamentally possible to use ambient light for the analysis, but such analyses show excessive errors due to fluctuations in the properties of the ambient light.
Analysis is therefore done using apparatuses that possess an illumination device for illuminating with optical radiation of given properties at least a part of a value document portion determined by a recording area of the apparatus, and a detection device for detecting optical radiation coming from the recording area, in particular a value document illuminated by the illumination device.
Although it is possible to use light sources such as halogen lamps for illumination, they consume a lot of power compared with the radiated power emitted in a desired spectral range and therefore require adequate cooling. They further have the disadvantage of not having a very long life. Furthermore, such light sources have considerable space requirements.